What size is ISS now?

[JonClarke]

The "people" are high level organisations responsible for metrology. Are they in error? For example, the National Institute of Standards and Technology clear lists pounds as a unit of mass in this document: http://physics.nist.gov/Document/sp811.pdf<br /><br />I repeat, all the high level sites of metrology list pound (avoirdupois) as an unit of mass. The use of pound as a unit of mass is clearly shown by compound units that measure heat content (BtU/lb), flow rate (lb/s) or power density (hp/lb). These are directly comparable to how the kg is used in equivalent units (J/kg, kg/s, and J/kg again). <br /><br />So far all we have is assertions that pounds is only unit of force. This is, quite simply, wrong. The conufsion arises because of the use of "pound" to mean both mass and force. If we want to be anal we should be differentiating between pound mass (or pounds avoirdupois) and pounds force - see http://scienceworld.wolfram.com/physics/Pound.html . This confusion is a good reason not to use either.<br /><br />This site has a useful discussion http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm#toc1a2<br /><br />Jon<br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[lampblack]

I have to observe -- quite simply -- that this is as <i>weighty</i> a thread as I have encountered here in quite some time. <div class="Discussion_UserSignature"> <font color="#0000ff"><strong>Just tell the truth and let the chips fall...</strong></font> </div>

 

[radarredux]

> <i><font color="yellow">What is your evidence that the pound is only a unit of force and not mass?</font>/i><br /><br />From my all too foggy recollection, a "pound" was a unit of weight, which is an amalgamation of mass and acceleration (i.e., gravity)... which is the definition of Force (F = ma). That is, a "pound" is a measure of "force" in the presense of gravity.<br /><br />The problem is that an object's "weight" is dependent the external factor of acceleration (i.e., of gravity), so an object that weighs 60 pounds on Earth weighs only 10 pounds on the Moon, weighs something else on Mars, etc. Whats more, the same object, measured in pounds, can weigh differently at different locations on Earth. The ISS sitting on the ground in Mexico would weigh differently than the ISS sitting in northern Canada -- that is, ISS would weigh X pounds in Mexico and Y pounds in Canada, where X != Y.<br /><br />But an object's mass remains constant everywhere (ignoring relativistic effects at high speeds).<br /><br />Having said all that, in common usage, "pound" is usually used interchangeably to describe both "force" and "mass" ("the engine delivers 20,000 pounds of thrust" or "I weigh 180 pounds"), the difference is usually understood through context. And in every day use by most people, it doesn't matter.</i>

 

[silylene old]

Jon - The unit named a "pound" is very confusing in its usage due to the ambiguities I think both you and I have pointed out in this thread. I <i>clearly</i> learned in my university Chem Engineering course to always use the term "slug" for mass, or I would receive a mark of zero. Fortunately, I was a chemistry major (which only uses SI or metric units) and only had to take one engineering course (English units).<br /><br /><font color="yellow">If we want to be anal we should be differentiating between pound mass (or pounds avoirdupois) and pounds force - see http://scienceworld.wolfram.com/physics/Pound.html . This confusion is a good reason not to use either. </font><br /><br />I wholeheartedly agree. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>

 

[JonClarke]

Exactly, that is the source of the confusion. Pound was originally a weight. When it was realised that weight was a force exerted by a mass under a particular accelleration they used to term pound for both the mass and the force. Probably unwise in retrospect. Which is why SI uses different terms for mass and force. However, pound is still legitimately a unit of mass. E.g. The encylopedic dictionary of physics (Pergamon, London, 1962): " POUND (MASS). The pound avoirdupois is the fundamental unit of mass in what is known as the Imperial system of measurement. " Or The international dictionary of physics and electronics (van Nostrand, Princeton): "POUND. A unit of mass in the English system." Both sources go on to differentiate the pound pass from the pound of force and other derived measures.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[drwayne]

The ambiguity was something one of the first Physics professors I ever had used to exploit to his advantage.<br /><br />He would give some measures, and ask which was a scalar, and which was a vector. I think you can guess where the term (pound) fell.<br /><br />If you go to the "Mass" section of the units conversions tables in the back of Halliday and Resnick (page A24 in my edition from about 30 years ago), they have a shaded area for which they state "Quantities in the shaded areas are not mass units but are often used as such. When we write for example 1 kg "=" 2.205 lb this means that a kilogram is a mass that weighs 2.205 pounds under the standard condition of gravity"<br /><br />Ambiguity due to history gives us several quirks, this one, and current definitions are two of the biggies...<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>